The present invention relates to the attenuation of sound, being conducted either through the air or through solid material, for purposes of soundproofing wheels, particularly gears.
Gears upon engaging, even under very normal conditions of operation, are subjected to a so-called insertion or engagement impact; as the feet engage there is an impact which stimulates vibrations, usually characteristic, natural vibrations, and in various modes.
These modes depend, of course, on the shape but also on the stiffness of the gear. The engagement, and, therefore, the stimulation depends on the configuration of the gearing. For example, in a typical case, a gear generally is comprised of the following separate elements; a hub, a gear disk, and gear rings, rims, or spurs.
The construction as outlined sets up a first resonance frequency at a mode which is equivalent to tilting the hub vis-a-vis the disk. The axis of rotation of the gear will then no longer be at right angles to the plane of the gear rim. This kind of oscillation produces primarily vibrations in the gear itself, and in solid parts being in physical connection therewith. This may be termed body sounds; sound generation in air is not very prevalent for this kind of mode.
In a second mode, the rim element or gear ring provides a periodic pumping motion in direction of the shaft, actually, in phase opposition but in two 90.degree. offset directions. This results in a periodic bending of the disk as between two 90.degree. offset ovals. Simultaneously, the hub is deformed, in accordance with two 90.degree. offset ovals, but these oscillations are only 90.degree. offset, so that in the case of a maximum deformation of the disks, in one direction, a maximum deformation of the hub in a 90.degree. angle obtains.
This particular mode, actually, is more important because for some reason it involves more energy and, therefore, produces more severe noise on and in the rim. It should be realized that increasing the stiffness of the disk, from an overall point of view, does not reduce the oscillations but rather shifts them towards higher frequencies. Hence, it is and remains necessary to attenuate the amplitude of the oscillation, at whatever frequency it occurs, in order to provide for a suppression of this particular kind of oscillation. This mode is, moreover, primarily responsible for body sound but also for sound eminating into and propagating through air.
A third mode involves a pumping motion of the hub in direction of the shaft in that three actions are superimposed. There is a strong bending of the disk, a lifting and tilting of the hub, and a pumping of the gear ring. In fact, four nodes are established therewith. Again, this mode is important and significant for body sound as well as sound propagating in and through air.
One can see from the description above that upon energizing and exciting these several modes, generally one obtains axial oscillations as well as radial ones. German printed patent application 31 41 101 suggests to provide a cover on both sides of the disk of the gear and with a visco-elastic attenuation layer is interposed, in order to obtain attenuation of the vibration of the gear. The description of this particular publication, page 1, last paragraph, as continued on page 2, first paragraph, reveals that this particular attenuation is provided just to eliminate and attenuate axial oscillations, but not radial ones. This contention made in the reference is believed to be correct.
A publication by Drago and Brown in PTD, July, 1979, particularly page 80, last paragraph, and FIG. 6 on that same page, suggests to provide attenuating rings, for example, clamping rings with square-shaped cross-section to be placed into grooves which are formed in the rim of a wheel so that attenuation obtains by operation of the centrifugal motion of the ring. Thus, attenuation is strictly a matter of the mass involved.